Extraordinary assumptions are assumptions that are:

• related to the appraisal assignment, 
• made as of the effective date of the appraisal, and 
• assumptions on which the appraiser bases his or her opinions or conclusions

An assumption that an item of property lost in a flood is sterling silver and not the less valuable silverplate is such an assumption. With extraordinary assumptions, the appraiser cannot verify that the assumption is true, but he believes that it is true based on the client's description and has no reason to doubt that it is not true. (This is as opposed to a hypothetical appraisal which, you will recall, is based on a hypothetical condition which is an assumption that the appraiser knows is not true at the time of the appraisal, but which forms the basis of the appraiser's conclusions, regardless.) If the extraordinary assumptions on which the appraiser relied turn out to be false, they could, in turn, cause the appraiser's opinions or conclusions to be in error.

Extraordinary assumptions may be called for in any type of appraisal, but they are particularly applicable in placing a value on property which is no longer available for the appraiser's inspection such as stolen property or property that has been destroyed by fire or flood. Extraordinary assumptions are critical when value is based on the use of client-provided descriptions and images, and not on a personal inspection. In such cases, the appraiser even needs to make the initial critical extraordinary assumption that the item existed in the first place, and that it was, indeed, owned by the client as claimed. Additional extraordinary assumptions as to quality of construction, condition, authorship, materials, etc. may also be required. Extraordinary assumptions must be disclosed within the appraisal report so the client and all other intended users of the appraisal report are aware on the basis for the appraiser's conclusions. Specifically, appraisals based on extraordinary assumptions must disclose:

• that the appraiser has used extraordinary assumptions as a basis for the reported opinions or conclusions, or portions thereof,
• the circumstances which called for the use of extraordinary assumptions, and
• the information that was provided to the appraiser on which the extraordinary assumption is based